1. Statement of the Technical Field
The invention concerns fiber optic devices, and more particularly, a method of filtering optical signals utilizing fiber optic devices having capillary waveguides.
2. Description of the Related Art
Optical fibers are used for data transmission within Dense Wavelength Division Multiplexed (DWDM) based networks. Devices within DWDM based networks transmit a number of signals through a single optical fiber. This is accomplished by transmitting each signal at a different wavelength along the longitudinal axis of the optical fiber. Consequently, DWDM based networks also include devices at a distal end of the optical fiber to filter a signal with a specific wavelength from a set of received signals. Tunable optical bandpass filters are often used for this purpose. In general, it is desirable for these filter devices to provide (1) wide tuning range, (2) negligible cross-talk with adjacent channels, (3) fast tuning speed, (4) low insertion loss, (5) polarization insensitivity and (6) stability with respect to environmental changes.
Various techniques are known for the purpose of implementing optical band pass filters. One such technique makes use of a fiber optic grating system. Fiber optic grating systems filter or scatter particular wavelengths of light propagating along the length of the fiber. Fiber gratings are formed by creating a periodic perturbation of the properties of the fiber. Typically, the periodic perturbation is with respect to the refractive index of the core. For example, a fiber optic grating system can include a core with a specific refractive index that varies along the length of the fiber.
Two basic types of grating systems which are known in the art include long period gratings and short period Bragg gratings. Short period fiber gratings are generally characterized as having a sub-micrometer period. These types of devices operate by coupling light from the forward propagating core mode to a backward propagating core mode. In general, the short period fiber Bragg grating will have selected narrow band reflection of specific wavelengths. Short period grating filters are commonly tuned by applying stress to the grating.
In contrast, long period gratings in optical fibers typically have a period in the range of a few tens of micrometers to 1 millimeter. Such long period grating promote coupling between forward propagating core modes and co-propagating cladding modes. Long period gratings generally attenuate a certain wavelength and offer wider bandwidths than short period gratings.
Various techniques have been proposed for tuning optical filters as described herein. For example, special materials have been used to form the cladding material surrounding the optical fiber core. These materials interact with optical energy extending into the cladding. Changing the index of refraction for these cladding materials tunes the frequency response of the device. One example of such a device is an optical fiber with air channels or capillaries in the cladding region of the fiber. The capillaries contain a fluid having a specific index of refraction. A long period grating is inscribed in the core which couples light of certain wavelengths from a forward propagating core mode into forward propagating cladding modes. Power at these wavelengths is attenuated. The response of the filter is tuned by moving the fluid into and out of the region where the grating is provided. Changing the position of the fluid in this way has been used to change the attenuation and/or wavelength of the transmission notch.
Other methods for tuning optical filters are also known. For example, a fiber can be physically stressed to modify its frequency response. Acousto-optic tunable filters use flexural waves propagating along a length of the fiber to tune the reflection wavelength of the optical grating. An electro-optic approach can also be used to modify the filter response. In particular, a long period grating can be formed in a core of a specialty fiber. A thin inner cladding of silica can be disposed over the core and an electro-optic outer cladding can be formed over the inner cladding. An applied voltage is used to change the refractive index of the outer cladding. This modification of the index of refraction of the outer cladding tunes the wavelength that is filtered.